A digital network comprises of a group of nodes that are connected to each other through a variety or interfaces. The network can also be logically considered to comprise several layers including, for example, a physical layer, a data link layer, a network layer, and a transport layer. In each of these layers, different agreed upon standards that enable various vendor equipment to communicate may be used. The standards are also known as communications protocols.
Asynchronous Transfer Mode (“ATM”) or “cell switching” is a technology designed for transmitting digital information such as voice, video, and data at high speeds through the digital network. In the ATM protocol, the digital information to be transferred is first arranged into equal sized units called cells having fixed lengths. The cells are then transmitted from node to node until they reach a destination node through a pathway (or connection) within the digital network.
The communication path between two nodes is established through a virtual circuit. In a virtual circuit, the path may be established and then removed, and multiple virtual circuits may share resources along the path. When the data cells are sent through network switches that established virtual circuits through an automated call-setup procedure, the communication paths are called Switched Virtual Circuits (“SVCs”).
The ATM Forum, which is a user and vendor group that establishes ATM standards, has defined several ATM class of service categories, used in characterization of virtual circuits, for example, (1) a Constant Bit Rate (CBR), which supports a constant or guaranteed rate to transport services, such as video or voice, as well as circuit emulation, which requires rigorous timing control and performance parameters; (2) a Variable Bit Rate (VBR), real time and non real time, which supports variable bit rate data traffic with average and peak traffic parameters; (3) an Available Bit Rate (ABR), which supports feedback to control the source rate in response to changed characteristics in the digital network; and (4) an Unspecified Bit Rate (UBR). Each class of service category has different guaranteed bandwidth requirements, delay, jitter, and cell loss parameters. Some class of service categories, such as, for example, ABR or UBR, rely mostly on best-effort service.
The digital network is constructed of digital switches coupled together through digital communication links such as, for example, trunks. The trunks carry the cells of information between the digital switches along the connection. The digital switches route the cells from incoming communication links to outgoing communication links and finally to the destination node.
In an ATM network, multicasting of cells has become an important function of the digital switches and it typically leads to efficient use of the network bandwidth. One method to multicast the traffic transmitted through a network node occurs at the egress unit level and involves input replication of the cells prior to queuing. An alternate method uses output replication of the cells subsequent to queuing of the cells. However, these methods can cause head of line blocking of the guaranteed traffic, such as CBR traffic, excessive delay due to congestion in a port, indiscriminate discarding operations at arrival due to queue build-ups and blocking.